The invention described herein relates to an improved system for collecting and transferring electrical current between relatively moving parts, as in a dynamoelectric machine.
The use of solid brushes for collecting or transferring current in systems involving relatively moving parts, such as motors or generators, has been proven reliable and reasonably efficient for many commercial and industrial applications. With the advent of the more recently advanced electrical machinery designs, particularly those involving superconducting excitation coils and high power rated homopolar machines, the need has arisen for improved systems capable of more efficiently collecting and transferring machine current. This need is primarily due to the much greater electrical currents and speeds required to generate more power or transmit more torque than was previously possible.
Present designs of solid brush current collectors operating with sliprings or commutator systems adequately handle current densities of about 10 amperes per square centimeter and brush lifetimes of 0.5-2 years are typical. These current densities apply to machines operating in ambient air and at conventional speeds but it is well known that brush lifetimes can be quadrupled by operating carbon brushes at current densities of about 10 amperes/cm.sup.2 in an inert gas atmosphere, such as the hydrogen environment utilized in large synchronous condensers.
Although the actual mechanisms or phenomena associated with current transfer across sliding surfaces are incompletely understood, it is known that the interface resistance (electrical base) and friction (mechanical base) between a slipring or a commutator bar and brushes and wear rates between the contacting members, are grossly affected by the type and reactivity of the gaseous atmosphere in which they operate, the temperature at which the contact members operate, particularly at the interface, and the properties of the contacting materials.
Concerning operation in a gaseous atmosphere, it is known that an unavoidable metal oxide film is deposited on collector surfaces during brush operation in atmospheric air. These brittle oxide films are semiconducting at best, and are physically hard and abrasive when disrupted during normal sliding operations. Because of this, they lead to relatively high unstable contact voltage drops and prevent achievement of optimum low brush friction and wear.
The deposition of such films on the collector surfaces can be minimized by operating the system in an inert gas atmosphere, rather than air. These oxygen-free environments which include carbon dioxide, sulfur hexafluoride and hydrogen, are effective in extending the carbon brush lifetimes and in lowering the contact voltage drops since the insulating and abrasively-hard tarnish films are avoided. However, the demand now exists for high current density brushes and the above gas environments were known to produce good results only at prevailing current densities, i.e. about 10 amperes per square centimeter. Also with regard to the environmental factor, the pressure and composition of ambient gases, including additives such as water vapor, contribute to the reduction of brush friction and wear. High friction and very high wear (dusting) occur when sliding contact pairs operate in vacuum or in dry gas ambients, such as at high altitudes.
The temperature at the brush-slipring interface also directly affects brush life since dusting will occur at predetermined temperatures for different carbon brush materials. It appears that desorption of moisture from the contacting surfaces becomes excessive as the critical temperature is reached for each humidity condition, and this condition must be eliminated for high current density applications.
It is therefore apparent that the need exists for an improved current collection system which will operate for greater lifetimes while simultaneously transferring current through the brushes in a magnitude 10 to 15 times greater than that possible in present designs.